In the prior art, open coil electrical resistance heaters are well known. These heaters employ a heater coil that is suspended or supported for electrical isolation by insulators, with the insulators themselves being supported by structure associated with the resistance heater. There are generally two types of insulators used in these types of heaters. One type is called “point suspension” type insulator, which is configured to engage convolutions of the coils for support. One problem with these types of insulators is that they are not adapted to easily support and engage coils with small diameters, small wire diameters, and/or small coil pitches (spacing between adjacent convolutions of the heater coil).
Typical prior art insulators are shown in FIG. 1 to illustrate the fit problems when the insulators are used with a small diameter heater coil. The insulators are shown engaging a heater coil with the insulators designated as 1 and 3 and the coil designated as 5. It can be seen in the areas 7 and 9 that the coil convolution spacing Y is severely altered when the insulators 1 and 3 engage the coil 5 and this causes problems over the life of the heater. Insulators like these can be found in U.S. Pat. No. 4,531,017 to Sherrill and U.S. Pat. No. 4,363,959 to Cottrell et al.
This fit problem can be solved using the conventional “string thru” type bushings. These bushings capture the coil by completely or partially surrounding it. One example is shown in FIG. 2, wherein the bushing 11 with its insulator 13 surrounds the coil 5. It can be seen that the coil pitch is not changed between the coil; it merely passes through the opening formed by the bushing. However, these types of insulators are problematic in that the coil 5 does not get full exposure to air flow when the coil 5 is used to heat air for a particular heating application. This is because the insulator 15 has a thickness that necessarily covers part of the coil.
Since heater applications are demanding new heater designs, which include coils with smaller diameters, smaller diameter wires, and smaller pitches, and the prior art insulators are ineffective for these types of heater coils, a need exists for improved insulators. The present invention responds to this need by providing an improved insulator for open coil electrical resistance heaters.